In automobile gasoline engines etc., so-called idle speed control devices (also termed `ISC devices`) for automatically controlling idle speeds to predetermined values are adopted for the purposes of the reduction of fuel costs, etc. in the mode of idling.
As described in, for example, the official gazette of Japanese Patent Application Laid-Open No. 57-195831 entitled "Idle speed control device," laid open on Dec. 1, 1982, the prior-art ISC devices are based on the feedback control with a control target value being the reference value of an idle speed in which the actual revolution speed of an engine in the idling mode is compared with the preset reference value of the idle speed so as to control the flow rate of intake air in accordance with the error between them. This feedback control has led to the disadvantage that, at the time of rapid deceleration of the engine in, e.g., the case where an accelerator pedal is suddenly returned, hunting is prone to occur, so a satisfactory controllability is not attained.